Black Walnut Trees: The Roots of Evil

Could this tree be toxic to your garden?

George and Becky Lohmiller

The black walnut tree is one of our most valuable and beautiful native trees. The easily worked, close-grained wood has long been prized by furniture- and cabinetmakers for its attractive color and exceptional durability. Its logs are in such demand for veneer that “walnut rustlers” have made off with trees in the dead of night and even used helicopters in their operations.

The early settlers discovered black walnuts growing in mixed forests from Canada to northern Florida and west to the Great Plains. They found that its rich-brown heartwood was exceptionally resistant to decay and put it to use as fence posts, poles, shingles, and sills. When surrounded by other trees in the forest, black walnut grows straight and tall with few, if any, lower branches. When planted in the open, the tree will branch out closer to the ground, developing a spreading shape that makes it easier to harvest its sweet, round, two- to three-inch nuts. The settlers snacked on the nutritious nuts out of hand, added them to soups and stews, and ground them into meal for baking; the hard shells provided a perfect package for storing the nuts over winter.

Unfortunately, the black walnut does have a dark side. Its roots, which may extend 50 feet or more from the trunk, exude a natural herbicide known as juglone that prevents many plants from growing within their reach. Tomatoes, potatoes, apples, pears, berries, and some landscape plants such as rhododendrons, azaleas, and lilacs may be killed or stunted if grown in close proximity to black walnut roots.

In spite of this, black walnuts make great shade trees for larger properties. They commonly grow to 50 feet or taller and about as wide, and specimens of more than 100 feet have been recorded. Black walnut’s large, fernlike foliage provides light, airy shade for those grasses and ground covers not affected by juglone. In autumn, the leaves turn bright yellow, contrasting nicely with the tree’s rugged, dark bark.

Black walnuts require a deep, fertile soil with a near-neutral or slightly acidic pH. They are pretty much disease-free and are threatened by few pests, with the exception of perhaps an occasional helicopter.


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Walnut Trees after El Nino

I have a wedding next June in a walnut grove. I'm wondering how El Nino and a wet winter/early spring will affect the look of the trees for the time of the wedding. Will the trees be green and leafy at that point? Or will it be bare?


Walnut Bliss

Hi, Melanie: To a small extent this depends on where you are, but it’s hard to imagine a scenario in which walnut trees are bare in June… You should be fine. Good luck!

I have a question I've been

I have a question I've been unable to find an answer to, as of yet.

I have three large black walnuts around the edge of my wooded property, but because of the overhang they drop over almost all of the large back yard. (It's dangerous to walk around come autumn.)
I -had- planned to build a small pond in my garden back there, but am now concerned (having recently learned about juglone and why so many rooted things were dying nearby) that the water will be constantly tainted by fallen, toxic debris. The pond was intended for not only aesthetics but to encourage tadpoles, turtles and give the local deer who frequent my yard a recirculating drinking hole. Bad idea?

Hi, PSG: This is a

Hi, PSG: This is a fascinating question. As you know, juglone is nasty stuff, but it is associated mainly with black walnut roots. Because it is not readily soluble in water, it does not travel well or far in soil. As you also know, it does indeed occur in the leaves, bark, twigs, etc., but in concentrations far lower than in the roots. Once it's exposed to air, bacteria, and/or water, juglone will deteriorate within 2 weeks to 2 months. Your question is about its effect on other organisms. To some extent, this would depend on the size (volume) of your pond; by the same token, if all of the "toxic debris" remains near the top, where the creatures are, then their exposure to it might be disproportionate to juglone's overall concentration level. Also, this might depend on the "order" of the species being affected, harbinger species such as frogs being much more susceptible to harm than deer, for example (which, if they get sick once, are not going to come back for a second sip). Is this pond a bad idea? Not necessarily. The juglone is not going to reach it through the soil. Have you thought about doing a small test pond first (or actually one small test pond divided in half physically, perhaps with netting over half) to see if you can see any difference in the flora/fauna? Good luck!

We have several black walnut

We have several black walnut trees and the nuts are turning and falling already. I am talking about a huge amount are coming down. What would cause this happen. We have had a rainy summer. And every tree is loaded.

Excessive rain during late

Excessive rain during late spring or early summer pollination causes poor or no pollination of female flowers. Then the nuts won't form or they will drop early as yours did. Pests and disease such as anthracnose can also cause trees to drop nuts early; you'd want to check the health of your tree. Good luck!

I live in Western North

I live in Western North Carolina, Asheville area. I have noticed several black walnuts are dropping the nuts alreay and its not near time. Any ideas, this is on several properties I see this happening? Thank you Laurel

Hi, Laurel: Early nut drop

Hi, Laurel: Early nut drop can be caused by any number of things, including the tree just trying to limit itself to the optimum amount of fruit. Assuming that disease and soil conditions aren't contributing to this, the most likely cause is often bad or insufficient pollination, one of the main causes of which can be a very wet spring. In your case, spring was rather dry--except for April, which had 2 inches more rain than normal, which is a lot. Let us know if you observe anything else!

I planted 2 English walnuts

I planted 2 English walnuts about 4 years ago they were bare root about 8' tall. they are about 15' tall now but all I ever get is tiny little what looks like walnuts in the spring. but they do not grow any bigger. Are they the walnuts that should get green and drop in Oct.? Thanks Larry

Hi, Larry: The most common

Hi, Larry: The most common cause of this is damage from spring cold or frost. If that is not a possibility and the trees otherwise seem healthy, the only thing to do is wait until they mature more. Sometimes an English walnut can take 6 to 8 years before it sort of "gets its feet under it" and begins to produce "normally." Thanks for asking!

We have a very mature (100+

We have a very mature (100+ years) black/english walnut tree. We are in the 4th year of a historic drought and I've noticed that this year, nearing the end of June, the tree has produced very few leaves. Normally it is leafed-out in May. Could this be a result of the drought and do you think there's a chance for recovery when rainfall normalizes? Thanks.

My neighbor has a large black

My neighbor has a large black walnut tree that is very close the our property line. There are been saplings that have turned into trees on our property line, destroying my fence. I continue to have saplings popping up in my yard, and have to pull them every week. I am very frustrated with this tree but understand my neighbors need for shade in Arizona. How far does the root system go down below ground level? I am considering digging a trench on the property line and putting a metal roofing or similar barrier to prohibit the roots traveling into my side of the yard and saplings appearing. However, I don't want to go through all the trouble and labor of digging it myself if the trees roots go very deep, and the barrier will do no good. If this barrier will do no good, what do you suggest? This tree is driving me crazy! Thank you so much.

We have 3 large black walnut

We have 3 large black walnut trees. This is the first year that they are dropping the newly forming nuts in the spring. We are a little concerned as to why this is happening. Can you help?

Hi, 3D: This could be from

Hi, 3D: This could be from any number of things, including some sort of infestation. Another thing that happens is that unkind weather conditions interfere with spring pollination, so the weakened tree starts dropping nuts. About all you can do is observe carefully as time goes on and see if anything seems to be afflicting the tree. Otherwise, hopefully it should be stronger next year.

Two years ago I had an old

Two years ago I had an old black walnut cut down and the stump ground up. I now would like to plant veggies there. Do I need to remove all the chips or can I mix in soil and plant that area. The mound of chips has shrunk by 3/4 over the past 2 years. Is it still toxic?

Hi,somewhere I read that you

Hi,somewhere I read that you can use black walnut for fence posts. Questions: does the toxicity apply to fence posts and if you cut down the tree, how do you destroy the toxicity?
Thank you.

Nice blog!yes black walnut is

Nice blog!yes black walnut is beautiful and herbal medicine tree.Dried Walnut Hull or its extract helps in relieving asthma, cough, and chronic bronchitis. Regular consumption of this herbal extract offers excellent relief for people of age suffering from chronic respiratory diseases. In Asian countries the use of black walnut and its extract a popular home remedy for treating respiratory diseases. Check out the Benefits of Black Walnut here: buytincture which is best herbal store.

We just purchased some land

We just purchased some land that is covered in black walnut trees. We will be taking some down to plant grass and do landscaping. How long are the roots toxic after they ate cut down?


Hi, Susan: The duration of

Hi, Susan: The duration of the toxicity of juglone, the poisonous agent of black walnut trees, really depends in some part on the type, density, and drainage of the soil (although juglone is not very soluble). Still, it can last for 5 or 6 years or even longer -- and it can be much shorter, too. There are two things to do: (1) When you "stump" (pull up the stumps on) your land, try to get as much of the big roots as possible, even if it means a little more work. They have a massive root system, but every little bit that's not there helps. (2) Research online or at your library what plants are tolerant of juglone -- there are quite a few: Kentucky blue grass, for example. Good luck!

I have a few acres of black

I have a few acres of black walnut trees. Unfortunately they were planted about 10 feet apart so they have grown tall and thin. They are about 50 feet tall with trunk diameters of 6-12inches. I'd like to thin some areas in hopes that the trees will branch out and spread at the crown. Is it too late for them or will opening the area encourage them to branch out?


Hi, Corinna: This is a good

Hi, Corinna: This is a good question, with answers so complicated that we dare not go there. Some of this depends on the purpose of the trees (harvest anything? shade? beauty?). In any event, walnut trees with this extremely tight spacing are best thinned when their diameters are 3 to 4 inches, not 6 and up. Your local climate and soil conditions might come into play, too, not to mention the extensive root systems. We recommend consulting with a professional arborist. Good luck!

Poison ivy vines are growing

Poison ivy vines are growing exclusively on all my walnut trees in the unkept, mature woods of the older home I just bought in northern MD. Wild grape, oak, and rosa flora vines are grwing on all the other trees but never on the walnuts. Why? I love and hate it. How agressive can I get with herbicides and not hurt the trees?

Hi, Mary: This seems to be a

Hi, Mary: This seems to be a question not only of arboriculture, but also of semantics. Why poison ivy is not growing on your other trees, we don't know. Why the grape, etc., are not growing on your black walnuts is because b-w's produce an herbicidal substance called juglone (see our intro above) that is toxic to many plants. It is not toxic to poison ivy, which is why p-i will grow on black walnuts. In and of itself, a good (p-i specific, if possible), organic herbicide is not going to kill the trees. In and of itself, it is also not going to kill the p-i vines. The thing to do is first to thoroughly cover every inch, nook, and cranny of yourself with protection from p-i's urushiol oil. Let us repeat that: Cover yourself totally. Then carefully remove (cut out) sections (a foot long is good) of the vines, which will kill what is above (but the oil will still be potent for a year or two). Then, focus on spraying what is below the cut. Carefully discard the cut sections in a way that no one can touch them or what they have touched. By the way, though: Poison ivy leaves are edible by quite a range of wildlife. It's a complicated situation, and no doubt this reply has just scratched the surface.

I just recently found out I

I just recently found out I have a black walnut tree in my next door neighbors home right on my fence line. It drops these green balls onto my roof and scares the crap out of me lolz...I grabbed one today threw it on my cement patio it broke the green ball and I seen it was a walnut shell on the inside. Now I live in northern illinois and my neighbors never planted the tree. How the heck is it surviving?

Normally, you need a special

Normally, you need a special tool to crack a black walnut shell (that is stronger than a common nut cracker) but I guess you found a creative solution! :-) Inside is a sweet nut that you can eat or make into pies.  The tree may have seeded itself; it's hard for us to know. Just keep in mind that the area near the tree can be toxic to other plants and vegetables, so something to keep in mind when you consider whether you want to keep it.

I have a whole yard full of

I have a whole yard full of black walnut trees and a garden right next to one and my plants don't get toxic as a matter of fact I had more tomatoes than I knew what to do with.

We have a Black walnut tree

We have a Black walnut tree in our front yard in Lincolnton, N.C. My question is we have been living here 5 years, and this is the first year it is not bearing nuts. Do they have a non-bearing year like the Pecan tree. Thank you

Some black walnut cultivars

Some black walnut cultivars are indeed prone to alternate bearing or just bearing more heavily every other year.
If your leaves seem unhealthy, here is also a fungus that can effect nut production but can be controlled by fungicide applications. You could contact your cooperative extension for further diagnosis.

My question is when you first

My question is when you first plant your black walnut tree and its only a foot tall,will the deer bite the tops off of them and kill the tree ?

I have or I think I have

I have or I think I have black wallnut trees on my prop. about 35 feet apart. For 15 years I,ve lost many pine trees that I've planted around these black wallnut trees. Is there a pine tree that resists the JUGLONE that was talked about?

I take exception to your

I take exception to your comment regarding Black Walnut pest.
You mention few are out there, we have 17 walnut trees in our back yard.
We live in S.W. Ohio 2.7 miles from a large Veneering company. They are suspect of importing diseased walnut trees from Colorado. We have a major infestation of Walnut Twig Beetles (size of a sesame seed) which carry a fungus which causes 1000 Canker Disease.
Our situation is so bad the state of Ohio Agriculture Dept. has conducted training classes in our yard. We've had Professors from Ohio State University and University of Tennessee investigating and taking samples. Plus the Ohio Division of Forestry Department had 25 of their personal here for observation.
We currently have one of in excess 110 traps around the county. These traps are not to catch the beetles for disposal rather they are for verification of the infestation in other areas.
It's now up to us to take down these dead or dying trees. Then we must figure out a means of disposal without removing any portion including saw dust from the county of which we live Butler County Ohio.
I decided to pass this information on in the event your Walnut trees decided to start dropping leaves early and often all summer long including.

Thanks for your comment. This

Thanks for your comment. This column was written in late 2005 or early 2006 and we appreciate your update.
The walnut twig beetle is a pest that seems to be increasing among black walnut trees (primarily affects black walnut Juglans nigra). Tinier than a grain of rice these beetles carry the fungus Geosmitha morbida that eventually (within a couple or few years) infects the tree and starves it to death. See:
Disposal of the remains can be a complicated and costly procedure.

Do you need to fertilize the

Do you need to fertilize the soil and also what pesticide do you need to treat the black walnut trees? Thanks

It would probably depend on

It would probably depend on whether the black walnuts grow naturally in your area. If they do, we'd say you needn't bother fertilizing, unless you are setting up a stand of trees to harvest the nuts or wood, or if the tree begins to look unhealthy. When fertilizing, we'd recommend that you test your soil for nutrients and then contact your county's Cooperative Extension or an arborist to see what fertilizer is needed for your local soil, for black walnuts, and for the age of the tree. Soil pH is best around 6.2 to 7.2.
The same would apply to pesticides. If the tree is native to your area, and you are just interested in the tree as a landscape feature, then it would not need pesticides unless a specific problem arises. If, however, you are planning to harvest, then monitoring for specific pests, such as the walnut husk fly, would be called for, with the appropriate pesticide that is safe for edibles if harvesting the nuts rather than the wood (check with an arborist or your local garden center for what's appropriate in your area).
For more information, you might be interested in the following:

I live in a neighborhood that

I live in a neighborhood that has many mature trees and I have mulitple black walnuts on my property. They are mature but I do not harvest them. The past few years I have been losing many large and small branch's some in storms but most are brought down by the slightest wind gusts. Is this normal? Is there something I can do to prevent it?

This can happen when there is

This can happen when there is too much weight on the ends of the branches. Just prune to thin your trees. Be careful not to overprune or strip the tree on one side or this can cause the tree to get weakened and the branches to bend and break, too.

I bought a large farm 2 years

I bought a large farm 2 years ago (2011) and dicovered about 8 fully grown walnut trees on the edge of the fields in the woods. I am going to try a harvest this year for pesonal consumption. It is august now and was wondering what a good time to and how do I know the best time for harvest. I am in Maryland.

Black walnuts are harvested

Black walnuts are harvested in the fall (mid to late September) once the hull changes from a solid green to a yellowish color and is soft enough that you can dent it with your thumb. The walnuts should ripen on the tree, so you can shake the tree or just wait until the nuts fall. Here is a good reference page on how to harvest and prepare the walnuts:

My neighbor has a huge Black

My neighbor has a huge Black Walnut on the border between our properties. It is exactly where it does us the least good, because it is overshadowing an area that would be perfect for a garden. I call it the "Black Widow Tree".

We have a very old very large

We have a very old very large black walnut. It is a lovely tree but drops yellow spotted leaves all summer. Keeps lawn covered and messy.